1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging, using a projection system, onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanning type lithographic apparatus, a reticle (patterning device) is coupled to a reticle stage. While generating a pattern on a target portion of a substrate, the reticle stage performs scanning movements along a line of movement, where the scan direction alternates between two successive scanning movements. For such a change of direction, it is desirable to decelerate and accelerate the reticle stage between the successive scanning movements. Also, it is desirable to accelerate and decelerate the reticle stage before and after each scanning movement in a specific direction. Conventionally, the scanning movements are made with constant velocity. However, the scanning movements may also at least partly be made with varying velocity, e.g. the movements including at least part of the deceleration and/or acceleration phases.
The reticle stage supports, i.e. bears the weight of, the reticle. It holds the reticle in a manner that depends on the orientation of the reticle, the design of the lithographic apparatus, and other conditions, such as for example whether or not the reticle is held in a vacuum environment. The reticle stage may comprise a frame or a table, for example, which may be fixed or movable as required. The reticle stage may ensure that the reticle is at a desired position, for example with respect to the projection system.
The reticle is coupled to the reticle stage through a coupling. Conventionally, the reticle is coupled to the reticle stage through a vacuum coupling which may be implemented as one or more vacuum pads provided on the reticle stage, where at least a part of a circumferential area of the reticle is held onto the vacuum pads. Thus, a normal force between adjacent surfaces of the reticle and the reticle stage is generated, resulting in a friction between contacting surfaces of the reticle and the reticle stage. The vacuum pads comprise one or more openings coupled to a gas discharge and supply system. At a discharge of gas, the part of the circumferential area of the reticle is held against the reticle stage, while at a supply of gas, the reticle is decoupled from the reticle stage, e.g. to exchange the reticle. Instead of a vacuum coupling between the reticle and the reticle stage, other forms of couplings based on friction between the reticle and the reticle stage are conceivable, such as electrostatic or mechanical clamping techniques to hold the reticle against the reticle stage.
In an ongoing development, increasing throughput requirements placed on lithographic apparatus lead to increasing scanning velocities. Consequently, deceleration and acceleration of the reticle stage increase. In the deceleration and acceleration phases, increased inertia forces act on the reticle stage and on the reticle.
It is known that inertia forces acting on the reticle stage and the reticle may lead to slip of the reticle and the reticle stage relative to each other. The slip usually is in the order of nanometers. For relatively low decelerations and accelerations, the slip has appeared to be low and approximately constant over time, changing its direction with each deceleration/acceleration phase. In such circumstances, the slip may be ignored if it is sufficiently low, or the slip may be compensated by suitably calibrating a positioning device controlling the position (and hence, the movement) of the reticle stage and/or the substrate stage.
However, with increasing decelerations and accelerations, the slip occurring between the reticle and the reticle stage increases, and becomes variable and unpredictable. Factors influencing the amount of slip may comprise a flatness and roughness of the surfaces of the reticle and the reticle stage engaging each other, a humidity of the atmosphere(s) in which the reticle and the reticle stage are handled, a contamination of the reticle or the reticle stage, and a degree of vacuum when the reticle is held on the reticle stage by vacuum pads. Thus, a calibration of the positioning device will not lead to a correct positioning of the reticle stage and/or the substrate stage under the circumstances of high inertia forces.